Skin test surveys of native American populations and of other populations potentially exposed to tularemia suggest much higher incidence of the disease than clinical histories would indicate. It is probable that many of the reactors suffered subclinical or undiagnosed tularemia, but antigenic relationships of Francisella tularensis and other organisms (F. novicida, Brucella sp.) pose the possibility that the skin test reaction may not be sufficiently specific for certain diagnosis. Cross reactivity in the skin test between those organisms is being studied with Dr. R. Philip. Stable quantum differences in susceptibility to tularemia infection and apparent failure to acquire immunity to the organism on enteric exposure have been noted. However, infection acquired via the gastro-intestinal tract is often chronic whereas even one organism of the strain used is acutely fatal when introduced parenterally. Effects of population (bacterial) "pressure" in the gut, and of associated flora, are being studied.